This invention relates to a continuous process for producing substantially homogeneous solid, non-tacky polymers of low moisture content by solution polymerization of acrylamide-containing monomer mixtures.
Acrylamide copolymers have been produced by a variety of techniques. For example, emulsion, suspension, solution or bulk polymerization techniques may be used. The preferred technique is solution polymerization. This may be effected in a batch process or, more desirably, in a continuous process.
Copending commonly assigned applications entitled "Aqueous Drilling Fluid Additive, Composition and Process", Ser. No. 946,227, filed Sept. 27, 1978 and "Copolymer Filtration Control Agent", Ser. No. 19,219, filed Mar. 9, 1979, both now abandoned in favor of continuing applications, disclose that certain polymers containing (meth)acrylamide or N-alkyl(meth)acrylamide are particularly effective filtration control agents for utilization in aqueous drilling fluids. In particular, a terpolymer of an alkali metal salt of acrylic acid, a hydroxyalkyl acrylate and acrylamide is advantageously used, as is a copolymer of a (meth)acrylamidoalkylsulfonic acid or alkali metal salt thereof and (meth)acrylamide or N-alkyl(meth)acrylamide, this copolymer being optionally cross-linked with a quaternary ammonium salt cross-linking agent. The copending applications disclose only batch processes for making these polymers. The present invention is a continuous process for the solution polymerization of these monomer mixtures to form solid, non-tacky polymers of low moisture content which are suitable, optionally after drying and milling, for use as filtration control agents in aqueous drilling fluids.
U.S. Pat. Nos. 3,663,518 and 3,732,193 disclose continuous processes wherein preformed solutions of acrylamide monomer mixtures and polymerization initiators are deposited on a moving belt and then heated to initiation temperature, after which they are passed into a polymerization zone. The process of U.S. Pat. No. 3,732,193 employs an inert gas atmosphere in the reaction zone.
U.S. Pat. No. 4,032,701 discloses a continuous process for producing a dry, solid polyacrylamide, wherein a concentrated aqueous solution of acrylamide monomer is heated to produce a single phase solution, and this solution is deposited, together with a separately prepared solution of initiator, on a heated rotating metal disc. The heated disc serves to further raise the temperature of the acrylamide/initiator mixture to a suitable initiation temperature, after which exothermic polymerization occurs to form a dry polymer.
These methods all require heating a mixture of monomers and initiator, after deposition on the moving surface, to reach the polymerization initiation temperature.
According to the method of U.S. Pat. Nos. 3,663,518 and 3,732,193, the initiator is added to the monomer solution at a relatively low temperature in the monomer tank. The mixture of monomer and initiator is then deposited on a moving surface and heated to the polymerization temperature on the moving surface. This is not entirely satisfactory for the commercial production of the polymers of this invention because either heat must be applied at such a high rate that very uneven heating occurs, with charring at the heated surface and some thermally initiated polymerization competing with free radical-initiated polymerization, or impractically long distances and/or thin reaction mixture films must be used, which increases equipment and production costs to an economically unacceptable level.
In the process of U.S. Pat. No. 4,032,701, the monomers would be heated in the monomer tank to 50.degree.-100.degree. C. This causes thermally initiated polymerization in the tank, at rates differing from the rates of free radical-initiated polymerization, which results in substantially non-uniform product and causes polymer buildup on the equipment. Competing thermally initiated and free radical-initiated polymerization is not a problem in the production of acrylamide homopolymer, which is the product made in U.S. Pat. No. 4,032,701, but would be a problem with applicant's monomer mixtures. This reference teaches heating the monomer solution until a single phase solution is achieved. In the case of the monomers used in the present invention, this temperature is about 30.degree. C., rather than about 50.degree. C. as in U.S. Pat. No. 4,032,701. Using the heated metal disc to raise the temperature of the mixture of monomers and initiator from about 30.degree. C. to about 70.degree. C., following the procedure of U.S. Pat. No. 4,032,701, would require either inordinately high temperatures for the disc, causing charring and very uneven heating, or an impractically large disc and thin reaction mixture film, again making this alternative economically unacceptable for commercial use.
Precise control of product composition, homogeneity and moisture content is not possible using the prior art continuous processes.